16k mode interleaver in a digital video broadcasting (DVB) standard

ABSTRACT

A data processing apparatus maps input symbols to be communicated onto a predetermined number of sub-carrier signals of an Orthogonal Frequency Division Multiplexed (OFDM) symbol. The data processor includes an interleaver memory which reads-in the predetermined number of data symbols for mapping onto the OFDM sub-carrier signals. The interleaver memory reads-out the data symbols on to the OFDM sub-carriers to effect the mapping, the read-out being in a different order than the read-in, the order being determined from a set of addresses, with the effect that the data symbols are interleaved on to the sub-carrier signals. The set of addresses are generated from an address generator which comprises a linear feedback shift register and a permutation circuit.

The present invention relates to data processing apparatus operable tomap input symbols onto sub-carrier signals of Orthogonal FrequencyDivision Multiplexed (OFDM) symbols.

The present invention also relates to data processing apparatus operableto map symbols received from a predetermined number of sub-carriersignals of OFDM symbols into an output symbol stream.

Embodiments of the present invention can provide an OFDMtransmitter/receiver.

BACKGROUND OF THE INVENTION

The Digital Video Broadcasting-Terrestrial standard (DVB-T) utilisesOrthogonal Frequency Division Multiplexing (OFDM) to communicate datarepresenting video images and sound to receivers via a broadcast radiocommunications signal. There are known to be two modes for the DVB-Tstandard which are known as the 2 k and the 8 k mode. The 2 k modeprovides 2048 sub-carriers whereas the 8 k mode provides 8192sub-carriers. Similarly for the Digital Video Broadcasting-Handheldstandard (DVB-H) a 4 k mode has been provided, in which the number ofsub-carriers is 4096.

In order to improve the integrity of data communicated using DVB-T orDVB-H a symbol interleaver is provided in order to interleave input datasymbols as these symbols are mapped onto the sub-carrier signals of anOFDM symbol. Such a symbol interleaver comprises an interleaver memoryin combination with an address generator. The address generatorgenerates an address for each of the input symbols, each addressindicating one of the sub-carrier signals of the OFDM symbol onto whichthe data symbol is to be mapped. For the 2 k mode and the 8 k mode anarrangement has been disclosed in the DVB-T standard for generating theaddresses for the mapping. Likewise for the 4 k mode of DVB-H standard,an arrangement for generating addresses for the mapping has beenprovided and an address generator for implementing this mapping isdisclosed in European Patent application 04251667.4. The addressgenerator comprises a linear feed back shift register which is operableto generate a pseudo random bit sequence and a permutation circuit. Thepermutation circuit permutes the order of the content of the linear feedback shift register in order to generate an address. The addressprovides an indication of one of the OFDM sub-carriers for carrying aninput data symbol stored in the interleaver memory, in order to map theinput symbols onto the sub-carrier signals of the OFDM symbol.Similarly, an address generator in the receiver is arranged to generateaddresses of the interleaver memory for storing the data symbolsreceived from the sub-carriers of OFDM symbols to read out the datasymbols to form an output data stream.

In accordance with a further development of the Digital VideoBroadcasting-Terrestrial broadcasting standard, known as DVB-T2 therehas been proposed that further modes for communicating data be provided.

SUMMARY OF INVENTION

According to an aspect of the present invention there is provided a dataprocessing apparatus operable to map input symbols to be communicatedonto a predetermined number of sub-carrier signals of an OrthogonalFrequency Division Multiplexed (OFDM) symbol. The data processingapparatus comprises an interleaver operable to read-into a memory thepredetermined number of data symbols for mapping onto the OFDMsub-carrier signals, and to read-out of the memory the data symbols forthe OFDM sub-carriers to effect the mapping. The read-out is in adifferent order than the read-in, the order being determined from a setof addresses, with the effect that the data symbols are interleaved onthe sub-carrier signals. The set of addresses is determined by anaddress generator, an address being generated for each of the inputsymbols to indicate one of the sub-carrier signals onto which the datasymbol is to be mapped.

The address generator comprises a linear feedback shift registerincluding a predetermined number of register stages and is operable togenerate a pseudo-random bit sequence in accordance with a generatorpolynomial, and a permutation circuit and a control unit. Thepermutation circuit is operable to receive the content of the shiftregister stages and to permute the bits present in the register stagesin accordance with a permutation order to form an address of one of theOFDM sub-carriers.

The control unit is operable in combination with an address checkcircuit to re-generate an address when a generated address exceeds apredetermined maximum valid address. The data processing apparatus ischaracterised in that the predetermined maximum valid address isapproximately sixteen thousand, the linear feedback shift register hasthirteen register stages with a generator polynomial for the linearfeedback shift register ofR′_(i)[12]=R′_(i−1)[0]⊕R′_(i−1)[1]⊕R′_(i−1)[4]⊕R′_(i−1)[5]⊕R′_(i−1)[9]⊕R′_(i−1)[11],and the permutation order forms, with an additional bit, a fourteen bitaddress R_(i)[n] for the i-th data symbol from the bit present in then-th register stage R′_(i)[n] in accordance with the table:

R′_(i) bit positions 12 11 10 9  8 7 6  5 4 3 2 1  0 R_(i) bit positions8 4 3 2  0 11 1  5 12 10 6 7  9

Although it is known within the DVB-T standard to provide the 2 k modeand the 8 k mode, and the DVB-H standard provides a 4 k mode, there hasbeen proposed to provide a 16 k mode for DVB-T2. Whilst the 8 k modeprovides an arrangement for establishing a single frequency network withsufficient guard periods to accommodate larger propagation delaysbetween DVB transmitters, the 2 k mode is known to provide an advantagein mobile applications. This is because the 2 k symbol period is onlyone quarter of the 8 k symbol period, allowing the channel estimation tobe more frequently updated allowing the receiver to track the timevariation of the channel due to doppler and other effects moreaccurately. The 2 k mode is therefore advantageous for mobileapplications.

In order to provide an even sparser deployment of DVB transmitterswithin a single frequency network, it has been proposed to provide the16 k mode. To implement the 16 k mode, a symbol interleaver must beprovided for mapping the input data symbols onto the sub-carrier signalsof the OFDM symbol.

Embodiments of the present invention can provide a data processingapparatus operable as a symbol interleaver for mapping data symbols tobe communicated on an OFDM symbol, having substantially sixteen thousandsub-carrier signals. In one embodiment the number of sub-carrier signalsmaybe a value substantially between twelve thousand and sixteen thousandthree hundred and eighty four, such as for example twelve thousand andninety six. Furthermore, the OFDM symbol may include pilot sub-carriers,which are arranged to carry known symbols, and the predetermined maximumvalid address depends on a number of the pilot sub-carrier symbolspresent in the OFDM symbol. As such the 16 k mode can be provided forexample for a DVB standard, such as DVB-T2, DVB-T or DVB-H.

Mapping data symbols to be transmitted onto the sub-carrier signals ofan OFDM symbol, where the number of sub-carrier signals is approximatelysixteen thousand, represents a technical problem requiring simulationanalysis and testing to establish an appropriate generator polynomialfor the linear feedback shift register and the permutation order. Thisis because the mapping requires that the symbols are interleaved ontothe sub-carrier signals with the effect that successive symbols from theinput data stream are separated in frequency by a greatest possibleamount in order to optimise the performance of error correction codingschemes.

Error correction coding schemes such as LDPC/BCH coding, which has beenproposed for DVB-T2 perform better when noise and degradation of thesymbol values resulting from communication is un-correlated. Terrestrialbroadcast channels may suffer from correlated fading in both the timeand the frequency domains. As such by separating encoded symbols on todifferent sub-carrier signals of the OFDM symbol by as much as possible,the performance of error correction coding schemes can be increased.

As will be explained, it has been discovered from simulation performanceanalysis that the generator polynomial for the linear feed back shiftregister in combination with the permutation circuit order indicatedabove, provides a good performance. Furthermore, by providing anarrangement which can implement address generating for each of the 2 kmode, the 4 k mode and the 8 k mode by changing the taps of thegenerator polynomial for the linear feed back shift register and thepermutation order, a cost effective implementation of the symbolinterleaver for the 16 k mode can be provided. Furthermore, atransmitter and a receiver can be changed between the 2 k mode, 4 kmode, 8 k mode and the 16 k mode by changing the generator polynomialand the permutation orders. This can be effected in software (or by theembedded signalling) whereby a flexible implementation is provided.

The additional bit, which is used to form the address from the contentof the linear feedback shift register, may be produced by a togglecircuit, which changes from 1 to 0 for each address, so as to reduce alikelihood that if an address exceeds the predetermined maximum validaddress, then the next address will be a valid address. In one examplethe additional bit is the most significant bit.

In one example the above permutation code is used to generate theaddresses for performing the interleaving for successive OFDM symbols.In other examples, the above permutation code is one of a plurality ofpermutation codes which are changed so as to reduce a possibility thatsuccessive or data bits which are close in order in an input data streamare mapped onto the same sub-carrier of an OFDM symbol. In one example,a different permutation code is used for performing the interleavingbetween successive OFDM symbols. The use of different permutation codesfor successive OFDM symbols can provide an advantage where the dataprocessing apparatus is operable to interleave the input data symbolsonto the sub-carrier signals of each of the OFDM symbols only by readingin the data symbols into the memory in a sequential order and readingout the data symbols from the memory in accordance with the set ofaddresses generated by the address generator.

Various aspects and features of the present invention are defined in theappended claims. Further aspects of the present invention include a dataprocessing apparatus and method operable to map symbols received from apredetermined number of sub-carrier signals of an Orthogonal FrequencyDivision Multiplexed (OFDM) symbol into an output symbol stream, as wellas a transmitter and a receiver.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings, wherein likeparts are provided with corresponding reference numerals, and in which:

FIG. 1 is a schematic block diagram of a Coded OFDM transmitter whichmay be used, for example, with the DVB-T2 standard;

FIG. 2 is a schematic block diagram of parts of the transmitter shown inFIG. 1 in which a symbol mapper and a frame builder illustrate theoperation of an interleaver;

FIG. 3 is a schematic block diagram of the symbol interleaver shown inFIG. 2;

FIG. 4 is a schematic block diagram of an interleaver memory shown inFIG. 3 and the corresponding symbol de-interleaver in the receiver;

FIG. 5 is a schematic block diagram of an address generator shown inFIG. 3 for the 16 k mode;

FIG. 6( a) is diagram illustrating results for an interleaver using theaddress generator shown in FIG. 5 for even symbols and FIG. 6( b) is adiagram illustrating design simulation results for odd symbols, whereasFIG. 6( c) is a diagram illustrating comparative results for an addressgenerator using a different permutation code for even and FIG. 6( d) isa corresponding diagram for odd symbols;

FIG. 7 is a schematic block diagram of a Coded OFDM receiver which maybe used, for example, with the DVB-T2 standard;

FIG. 8 is a schematic block diagram of a symbol de-interleaver whichappears in FIG. 7;

FIG. 9( a) is diagram illustrating results for an interleaver using theaddress generator shown in FIG. 5 for even OFDM symbols and FIG. 9( b)is a diagram illustrating results for odd OFDM symbols. FIGS. 9( a) and9(b) show plots of the distance at the interleaver output ofsub-carriers that were adjacent at the interleaver input;

FIG. 10 provides a schematic block diagram of the symbol interleavershown in FIG. 3, illustrating an operating mode in which interleaving isperformed in accordance with an odd interleaving mode only; and

FIG. 11 provides a schematic block diagram of the symbol de-interleavershown in FIG. 8, illustrating the operating mode in which interleavingis performed in accordance with the odd interleaving mode only.

DESCRIPTION OF PREFERRED EMBODIMENTS

It has been proposed that the number of modes, which are availablewithin the DVB-T2 standard should be extended to include a 1 k mode, a16 k mode and a 32 k mode. The following description is provided toillustrate the operation of a symbol interleaver in accordance with thepresent technique, although it will be appreciated that the symbolinterleaver can be used with other modes and other DVB standards.

FIG. 1 provides an example block diagram of a Coded OFDM transmitterwhich may be used for example to transmit video images and audio signalsin accordance with the DVB-T2 standard. In FIG. 1 a program sourcegenerates data to be transmitted by the COFDM transmitter. A video coder2, and audio coder 4 and a data coder 6 generate video, audio and otherdata to be transmitted which are fed to a program multiplexer 10. Theoutput of the program multiplexer 10 forms a multiplexed stream withother information required to communicate the video, audio and otherdata. The multiplexer 10 provides a stream on a connecting channel 12.There may be many such multiplexed streams which are fed into differentbranches A, B etc. For simplicity, only branch A will be described.

As shown in FIG. 1 a Coded Orthogonal Frequency Division Multiplexing[COFDM]transmitter receives the stream at a multiplexer adaptation andenergy dispersal block 22. The multiplexer adaptation and energydispersal block 22 randomizes the data and feeds the appropriate data toa forward error correction encoder 24 which performs error correctionencoding of the stream. A bit interleaver 26 is provided to interleavethe encoded data bits which for the example of DVB-T2 is the Low-DensityParity Check/Bose-Chaudhuri-Hocquengham (LDCP/BCH) encoder output. Theoutput from the bit interleaver 26 is fed to a bit into constellationmapper 28, which maps groups of bits onto a constellation point, whichis to be used for conveying the encoded data bits. The outputs from thebit into constellation mapper 28 are constellation point labels thatrepresent real and imaginary components. The constellation point labelsrepresent data symbols formed from two or more bits depending on themodulation scheme used. These will be referred to as data cells. Thesedata cells are passed through a time-interleaver 30 whose effect is tointerleaver data cells resulting from multiple LDPC code words.

The data cells are received by a frame builder 32, with data cellsproduced by branch B etc in FIG. 1, via other channels 31. The framebuilder 32 then forms many data cells into sequences to be conveyed onCOFDM symbols, where a COFDM symbol comprises a number of data cells,each data cell being mapped onto one of the sub-carriers. The number ofsub-carriers will depend on the mode of operation of the system, whichmay include one of 1k, 2k, 4k, 8k, 16k or 32k, each of which provides adifferent number of sub-carriers according, for example, to thefollowing table:

Number of Sub-carriers Adapted from DVB-T/H Mode Sub-carriers 1 k 756 2k 1512 4 k 3024 8 k 6048 16 k  12096 32 k  24192

Thus in one example, the number of sub-carriers for the 16 k mode istwelve thousand and ninety six. For the DVB-T2 system, the number ofsub-carriers per OFDM symbol can vary depending upon the number of pilotand other reserved carriers. Thus, in DVB-T2, unlike in DVB-T, thenumber of sub-carriers for carrying data is not fixed. Broadcasters canselect one of the operating modes from 1 k, 2 k, 4 k, 8 k, 16 k, 32 keach providing a range of sub-carriers for data per OFDM symbol, themaximum available for each of these modes being 1024, 2048, 4096, 8192,16384, 32768 respectively. In DVB-T2 a physical layer frame is composedof many OFDM symbols. Typically the frame starts with one or morepreamble or P2 OFDM symbols, which are then followed by a number payloadcarrying OFDM symbols. The end of the physical layer frame is marked bya frame closing symbols. For each operating mode, the number ofsub-carriers may be different for each type of symbol. Furthermore, thismay vary for each according to whether bandwidth extension is selected,whether tone reservation is enabled and according to which pilotsub-carrier pattern has been selected. As such a generalisation to aspecific number of sub-carriers per OFDM symbol is difficult. However,the frequency interleaver for each mode can interleave any symbol whosenumber of sub-carriers is smaller than or the same as the maximumavailable number of sub-carriers for the given mode. For example, in the1 k mode, the interleaver would work for symbols with the number ofsub-carriers being less than or equal to 1024 and for 16 k mode, withthe number of sub-carriers being less than or equal to 16384. Thesequence of data cells to be carried in each COFDM symbol is then passedto the symbol interleaver 33. The COFDM symbol is then generated by aCOFDM symbol builder block 37 which introduces pilot and synchronisingsignals fed from a pilot and embedded signal former 36. An OFDMmodulator 38 then forms the OFDM symbol in the time domain which is fedto a guard insertion processor 40 for generating a guard intervalbetween symbols, and then to a digital to analogue convertor 42 andfinally to an RF amplifier within an RF frontend 44 for eventualbroadcast by the COFDM transmitter from an antenna 46.

Providing a 16 k Mode

To create a new 16K mode, several elements are to be defined, one ofwhich is the 16K symbol interleaver 33. The bit to constellation mapper28, symbol interleaver 33 and the frame builder 32 are shown in moredetail in FIG. 2.

As explained above, the present invention provides a facility forproviding a quasi-optimal mapping of the data symbols onto the OFDMsub-carrier signals. According to the example technique the symbolinterleaver is provided to effect the optimal mapping of input datasymbols onto COFDM sub-carrier signals in accordance with a permutationcode and generator polynomial, which has been verified by simulationanalysis.

As shown in FIG. 2 a more detailed example illustration of the bit tosymbol constellation mapper 28 and the frame builder 32 is provided toillustrate an example embodiment of the present technique. Data bitsreceived from the bit interleaver 26 via a channel 62 are grouped intosets of bits to be mapped onto a data cell, in accordance with a numberof bits per symbol provided by the modulation scheme. The groups ofbits, which forms a data word, are fed in parallel via data channels 64the a mapping processor 66. The mapping processor 66 then selects one ofthe data symbols, in accordance with a pre-assigned mapping. Theconstellation point, is represented by a real and an imaginary componentthat is provided to the output channel 29 as one of a set of inputs tothe frame builder 32.

The frame builder 32 receives the data cells from the bit toconstellation mapper 28 through channel 29, together with data cellsfrom the other channels 31. After building a frame of many COFDM cellsequences, the cells of each COFDM symbol are then written into aninterleaver memory 100 and read out of the interleaver memory 100 inaccordance with write addresses and read addresses generated by anaddress generator 102. According to the write-in and read-out order,interleaving of the data cells is achieved, by generating appropriateaddresses. The operation of the address generator 102 and theinterleaver memory 100 will be described in more detail shortly withreference to FIGS. 3, 4 and 5. The interleaved data cells are thencombined with pilot and synchronisation symbols received from the pilotand embedded signalling former 36 into an OFDM symbol builder 37, toform the COFDM symbol, which is fed to the OFDM modulator 38 asexplained above.

Interleaver

FIG. 3 provides an example of parts of the symbol interleaver 33, whichillustrates the present technique for interleaving symbols. In FIG. 3the input data cells from the frame builder 32 are written into theinterleaver memory 100. The data cells are written into the interleavermemory 100 according to a write address fed from the address generator102 on channel 104, and read out from the interleaver memory 100according to a read address fed from the address generator 102 on achannel 106. The address generator 102 generates the write address andthe read address as explained below, depending on whether the COFDMsymbol is odd or even, which is identified from a signal fed from achannel 108, and depending on a selected mode, which is identified froma signal fed from a channel 110. As explained, the mode can be one of a1 k mode, 2 k mode, 4 k mode, 8 k mode, 16 k mode or a 32 k mode. Asexplained below, the write address and the read address are generateddifferently for odd and even symbols as explained with reference to FIG.4, which provides an example implementation of the interleaver memory100.

In the example shown in FIG. 4, the interleaver memory is shown tocomprise an upper part 100 illustrating the operation of the interleavermemory in the transmitter and a lower part 340, which illustrates theoperation of the de-interleaver memory in the receiver. The interleaver100 and the de-interleaver 340 are shown together in FIG. 4 in order tofacilitate understanding of their operation. As shown in FIG. 4 arepresentation of the communication between the interleaver 100 and thede-interleaver 340 via other devices and via a transmission channel hasbeen simplified and represented as a section 140 between the interleaver100 and the de-interleaver 340. The operation of the interleaver 100 isdescribed in the following paragraphs:

Although FIG. 4 provides an illustration of only four input data cellsonto an example of four sub-carrier signals of a COFDM symbol, it willbe appreciated that the technique illustrated in FIG. 4 can be extendedto a larger number of sub-carriers such as 756 for the 1 k mode 1512 forthe 2 k mode, 3024 for the 4 k mode and 6048 for the 8 k mode, 12096 forthe 16 k mode and 24192 for the 32 k mode.

The input and output addressing of the interleaver memory 100 shown inFIG. 4 is shown for odd and even symbols. For an even COFDM symbol thedata cells are taken from the input channel and written into theinterleaver memory 124.1 in accordance with a sequence of addresses 120generated for each COFDM symbol by the address generator 102. The writeaddresses are applied for the even symbol so that as illustratedinterleaving is effected by the shuffling of the write-in addresses.Therefore, for each interleaved symbol y(h(q)) =y′(q).

For odd symbols the same interleaver memory 124.2 is used. However, asshown in FIG. 4 for the odd symbol the write-in order 132 is in the sameaddress sequence used to read out the previous even symbol 126. Thisfeature allows the odd and even symbol interleaver implementations toonly use one interleaver memory 100 provided the read-out operation fora given address is performed before the write-in operation. The datacells written into the interleaver memory 124.2 during odd symbols arethen read out in a sequence 134 generated by the address generator 102for the next even COFDM symbol and so on. Thus only one address isgenerated per symbol, with the read-in and write-out for the odd/evenCOFDM symbol being performed contemporaneously.

In summary, as represented in FIG. 4, once the set of addresses H(q) hasbeen calculated for all active sub-carriers, the input vector Y′=(y′,y′₁,y′₂, . . . y′_(Nmax−1)) is processed to produce the interleavedvector Y =(y₀, y₁, Y₂, . . . y_(Nmax−1)) defined by:

-   -   y_(H(q))=y′_(q) for even symbols for q=0, . . . , N_(max)−1    -   y_(q)=y′_(H(q)) for odd symbols for q=0, . . . , N_(max)−1

In other words, for even OFDM symbols the input words are written in apermutated way into a memory and read back in a sequential way, whereasfor odd symbols, they are written sequentially and read back permutated.In the above case, the permutation H(q) is defined by the followingtable:

TABLE 1 permutation for simple case where Nmax = 4 q 0 1 2 3 H(q) 1 3 02

As shown in FIG. 4, the de-interleaver 340 operates to reverse theinterleaving applied by the interleaver 100, by applying the same set ofaddresses as generated by an equivalent address generator, but applyingthe write-in and read-out addresses in reverse. As such, for evensymbols, the write-in addresses 342 are in sequential order, whereas theread out address 344 are provided by the address generator.Correspondingly, for the odd symbols, the write-in order 346 isdetermined from the set of addresses generated by the address generator,whereas read out 348 is in sequential order.

Address Generation for the 16 k Mode

A schematic block diagram of the algorithm used to generate thepermutation function H(q) is represented in FIG. 5 for the 16K mode.

An implementation of the address generator 102 for the 16 k mode isshown in FIG. 5. In FIG. 5 a linear feed back shift register is formedby thirteen register stages 200 and a xor-gate 202 which is connected tothe stages of the shift register 200 in accordance with a generatorpolynomial. Therefore, in accordance with the content of the shiftregister 200 a next bit of the shift register is provided from theoutput of the xor-gate 202 by xoring the content of shift registersR[0], R[1], R[4], R[5], R[9], R[11] according to the generatorpolynomial:R′ _(i)[12]=R′ _(i−1)[0]⊕R′ _(i−1)[1]⊕R′ _(i−1)[4]⊕R′ _(i−1)[5]⊕R′_(i−1)[9]⊕R′ _(i−1)[11]

According to the generator polynomial a pseudo random bit sequence isgenerated from the content of the shift register 200. However, in orderto generate an address for the 16k mode as illustrated, a permutationcircuit 210 is provided which effectively permutes the order of the bitswithin the shift register 200 from an order R′_(i)[n] to an orderR_(i)[n] at the output of the permutation circuit 210. Thirteen bitsfrom the output of the permutation circuit 210 are then fed on aconnecting channel 212 to which is added a most significant bit via achannel 214 which is provided by a toggle circuit 218. A fourteen bitaddress is therefore generated on channel 212. However, in order toensure the authenticity of an address, an address check circuit 216analyses the generated address to determine whether it exceeds apredetermined maximum value. The predetermined maximum value maycorrespond to the maximum number of sub-carrier signals, which areavailable for data symbols within the COFDM symbol, available for themode which is being used. However, the interleaver for the 16k mode mayalso be used for other modes, so that the address generator 102 may alsobe used for the 2k mode, 4k mode, 8k mode, 16k mode and the 32k mode, byadjusting accordingly the number of the maximum valid address.

If the generated address exceeds the predetermined maximum value then acontrol signal is generated by the address check unit 216 and fed via aconnecting channel 220 to a control unit 224. If the generated addressexceeds the predetermined maximum value then this address is rejectedand a new address regenerated for the particular symbol.

For the 16 k mode, an (N_(r)−1) bit word R′_(i) is defined, withN_(r)=log₂ M_(max), where M_(max)=16384 using a LFSR (Linear FeedbackShift Register).

The polynomials used to generate this sequence is:16K mode: R′ _(i)[12]=R′ _(i−1)[0]⊕R′ _(i−1)[1]⊕R′ _(i−1)[4]⊕R′_(i−1)[5]⊕R′ _(i−1)[9]⊕R′ _(i−1)[11]

where i varies from 0 to M_(max)−1

Once one R′_(i), word has been generated, the R′_(i), word goes througha permutation to produce another (N_(r)−1) bit word called R_(i). R_(i)is derived from R′_(i) by the bit permutations given as follows:

Bit permutation for the 16K mode R′_(i) bit positions 12 11 10 9  8 7 6 5 4 3 2  1  0 R_(i) bit positions 8 4 3 2  0 11 1  5 12 10 6  7  9

As an example, this means that for the mode 16K, the bit number 12 ofR′_(i) is sent in bit position number 8 of R_(i).

The address H(q) is then derived from R_(i) through the followingequation:

${H(q)} = {{\left( {i\;{mod}\; 2} \right) \cdot 2^{N_{r} - 1}} + {\sum\limits_{j = 0}^{N_{r} - 2}{{R_{i}(j)} \cdot 2^{j}}}}$

The (i mod2)·2^(N) ^(r) ⁻¹ part of the above equation is represented inFIG. 5 by the toggle block T 218.

An address check is then performed on H(q) to verify that the generatedaddress is within the range of acceptable addresses: if (H(q)<N_(max)),where N_(max)=12096 for example in the 16K mode, then the address isvalid. If the address is not valid, the control unit is informed and itwill try to generate a new H(q) by incrementing the index i.

The role of the toggle block is to make sure that we do not generate anaddress exceeding N_(max) twice in a row. In effect, if an exceedingvalue was generated, this means that the MSB (i.e. the toggle bit) ofthe address H(q) was one. So the next value generated will have a MSBset to zero, insuring to produce a valid address.

The following equations sum up the overall behaviour and help tounderstand the loop structure of this algorithm:

  q = 0;   for  (i = 0; i < M_(max); i = i + 1)$\left\{ {{{H(q)} = {{\left( {i{mod2}} \right) \cdot 2^{N_{r} - 1}} + {\sum\limits_{j = 0}^{N_{r} - 2}{{R_{i}(j)} \cdot 2^{j}}}}};{{{if}\mspace{14mu}\left( {{H(q)} < N_{\max}} \right)q} = {q + 1}};} \right\}$

As will be explained shortly, in one example of the address generator,the above mentioned permutation code is used for generating addressesfor all OFDM symbols. In another example, the permutation codes may bechanged between symbols, with the effect that a set of permutation codesare cycled through for successive OFMD symbols. To this end, the controllines 108, 110 providing an indication as to whether the OFDM symbol isodd or even and the current mode are used to select the permutationcode. This example mode in which a plurality of permutation codes arecycled through is particularly appropriate for the example in which theodd interleaver only is used, which will be explained later. A signalindicating that a different permutation code should be used is providedvia a control channel 111. In one example the possible permutation codesare pre-stored in the permutation code circuit 210. In another example,the control unit 224 supplies the new permutation code to be used for anOFDM symbol.

Analysis Supporting the Address Generator for the 16 k Mode

The selection of the polynomial generator and the permutation codeexplained above for the address generator 102 for the 16 k mode has beenidentified following simulation analysis of the relative performance ofthe interleaver. The relative performance of the interleaver has beenevaluated using a relative ability of the interleaver to separatesuccessive symbols or an “interleaving quality”. As mentioned above,effectively the interleaving must perform for both odd and even symbols,in order to use a single interleaver memory. The relative measure of theinterleaver quality is determined by defining a distance D (in number ofsub-carriers). A criterion C is chosen to identify a number ofsub-carriers that are at distance≦D at the output of the interleaverthat were at distance≦D at the input of the interleaver, the number ofsub-carriers for each distance D then being weighted with respect to therelative distance. The criterion C is evaluated for both odd and evenCOFDM symbols. Minimising C produces a superior quality interleaver.

$C = {{\sum\limits_{1}^{d = D}{{N_{even}(d)}/d}} + {\sum\limits_{1}^{d = D}{{N_{odd}(d)}/d}}}$

where: N_(even)(d) and N_(odd)(d) are number of sub-carriers in an evenand odd symbol respectively at the output of the interleaver that remainwithin d sub-carrier spacing of each other.

Analysis of the interleaver identified above for the 16 k mode for avalue of D=5 is shown in FIG. 6( a) for the even COFDM symbols and inFIG. 6( b) for the odd COFDM symbol. According to the above analysis,the value of C for the permutation code identified above for the 16 kmode produced a value of C=22.43, that the weighted number ofsub-carriers with symbols which are separated by five or less in theoutput according to the above equation was 22.43.

A corresponding analysis is provided for an alternative permutation codefor even COFDM symbols in FIG. 6( c) for odd COFDM symbols in FIG. 6(d). As can be seen in comparison to the results illustrated in FIGS. 6(a) and 6(b), there are more components present which represent symbolsseparated by small distances such as D=1, and D=2, when compared withthe results shown in FIGS. 6( a) and 6(b), illustrating that thepermutation code identified above for the 16 k mode symbol interleaverproduces a superior quality interleaver.

Alternative Permutation Codes

The following nine alternative possible codes ([n]R_(i) bit positions,where n=1 to 9) have been found to provide a symbol interleaver with agood quality as determined by the criterion C identified above.

Bit permutation for the 16K mode R′_(i) bit positions 12 11 10 9 8 7 6 54 3 2 1 0 [1]R_(i) bit positions 7 12 5 8 9 1 2 3 4 10 6 11 0 [2]R_(i)bit positions 8 5 4 9 2 3 0 1 6 11 7 12 10 [3]R_(i) bit positions 7 5 69 11 2 3 0 8 4 1 12 10 [4]R_(i) bit positions 11 5 10 4 2 1 0 7 12 8 9 63 [5]R_(i) bit positions 3 9 4 10 0 6 1 5 8 11 7 2 12 [6]R_(i) bitpositions 4 6 3 2 0 7 1 5 8 10 12 9 11 [7]R_(i) bit positions 10 4 3 2 18 0 6 7 9 11 5 12 [8]R_(i) bit positions 10 4 11 3 7 1 5 0 2 12 8 6 9[9]R_(i) bit positions 2 4 11 9 0 10 1 7 8 6 12 3 5Receiver

FIG. 7 provides an example illustration of a receiver which may be usedwith the present technique. As shown in FIG. 7, a COFDM signal isreceived by an antenna 300 and detected by a tuner 302 and convertedinto a digital form by an analogue-to-digital converter 304. A guardinterval removal processor 306 removes the guard interval from areceived COFDM symbol, before the data is recovered from the COFDMsymbol using a Fast Fourier Transform (FFT) processor 308 in combinationwith a channel estimator and correction 310 in co-operation with aembedded-signalling decoding unit 311, in accordance with knowntechniques. The demodulated data is recovered from a mapper 312 and fedto a symbol de-interleaver 314, which operates to effect the reversemapping of the received data symbol to re-generate an output data streamwith the data de-interleaved.

The symbol de-interleaver 314 is formed from a data processing apparatusas shown in FIG. 8 with an interleaver memory 540 and an addressgenerator 542. The interleaver memory is as shown in FIG. 4 and operatesas already explained above to effect de-interleaving by utilizing setsof addresses generated by the address generator 542. The addressgenerator 542 is formed as shown in FIG. 8 and is arranged to generatecorresponding addresses to map the data symbols recovered from eachCOFDM sub-carrier signals into an output data stream.

The remaining parts of the COFDM receiver shown in FIG. 7, including theBit De-Interleaver 316, are provided to effect error correction decoding318 to correct errors and recover an estimate of the source data.

One advantage provided by the present technique for both the receiverand the transmitter is that a symbol interleaver and a symbolde-interleaver operating in the receivers and transmitters can beswitched between the 1 k, 2 k, 4 k, 8 k, 16 k and the 32 k mode bychanging the generator polynomials and the permutation order. Hence theaddress generator 542 shown in FIG. 8 includes an input 544, providingan indication of the mode as well as an input 546 indicating whetherthere are odd/even COFDM symbols. A flexible implementation is therebyprovided because a symbol interleaver and de-interleaver can be formedas shown in FIGS. 3 and 8, with an address generator as illustrated ineither of FIG. 5. The address generator can therefore be adapted to thedifferent modes by changing to the generator polynomials and thepermutation orders indicated for each of the modes. For example, thiscan be effected using a software change. Alternatively, in otherembodiments, an embedded signal indicating the mode of the DVB-T2transmission can be detected in the receiver in the embedded-signallingprocessing unit 311 and used to configure automatically the symbolde-interleaver in accordance with the detected mode.

Optimal Use of Odd Interleavers

As shown in FIG. 4, two symbol interleaving processes, one for evenCOFDM symbols and one for odd COFDM symbols allows the amount of memoryused during interleaving to be reduced. In the example shown in FIG. 4,the write in order for the odd symbol is the same as the read out orderfor the even symbol therefore, while an odd symbol is being read fromthe memory, an even symbol can be written to the location just readfrom; subsequently, when that even symbol is read from the memory, thefollowing odd symbol can be written to the location just read from.

As mentioned above, during an experimental analysis of the performanceof the interleavers (using criterion C as defined above) and for exampleshown in FIG. 9( a) and FIG. 9( b) it has been discovered that theinterleaving schemes designed for the 2 k and 8 k symbol interleaversfor DVB-T and the 4 k symbol interleaver for DVB-H work better for oddsymbols than even symbols. Thus from performance evaluation results ofthe interleavers, for example, as illustrated by FIGS. 9( a) and 9(b)have revealed that the odd interleavers work better than the eveninterleavers. This can be seen by comparing FIG. 9( a) which showsresults for an interleaver for even symbols and FIG. 6( b) illustratingresults for odd symbols: it can be seen that the average distance at theinterleaver output of sub-carriers that were adjacent at the interleaverinput is greater for an interleaver for odd symbols than an interleaverfor even symbols.

As will be understood, the amount of interleaver memory required toimplement a symbol interleaver is dependent on the number of datasymbols to be mapped onto the COFDM carrier symbols. Thus a 16 k modesymbol interleaver requires half the memory required to implement a 32 kmode symbol interleaver and similarly, the amount of memory required toimplement an 8 k symbol interleaver is half that required to implement a16 k interleaver. Therefore a transmitter or receiver which is arrangedto implement a symbol interleaver of a mode, which sets the maximumnumber of data symbols which can be carried per OFDM symbol, then thatreceiver or transmitter will include sufficient memory to implement twoodd interleaving processes for any other mode, which provides half orsmaller than half the number of sub-carriers per OFDM symbol in thatgiven maximum mode. For example a receiver or transmitter including a32K interleaver will have enough memory to accommodate two 16K oddinterleaving processes each with their own 16K memory.

Therefore, in order to exploit the better performance of the oddinterleaving processes, a symbol interleaver capable of accommodatingmultiple modulation modes can be arranged so that only an odd symbolinterleaving process is used if in a mode which comprises half or lessthan half of the number of sub-carriers in a maximum mode, whichrepresents the maximum number of sub-carriers per OFDM symbol. Thismaximum mode therefore sets the maximum memory size. For example, in atransmitter/receiver capable of the 32K mode, when operating in a modewith fewer carriers (i.e. 16K, 8K, 4K or 1K) then rather than employingseparate odd and even symbol interleaving processes, two oddinterleavers would be used.

An illustration of an adaptation of the symbol interleaver 33 which isshown in FIG. 3 when interleaving input data symbols onto thesub-carriers of OFDM symbols in the odd interleaving mode only is shownin FIG. 10. The symbol interleaver 33.1 corresponds exactly to thesymbol interleaver 33 as shown in FIG. 3, except that the addressgenerator 102 is adapted to perform the odd interleaving process only.For the example shown in FIG. 10, the symbol interleaver 33.1 isoperating in a mode where the number of data symbols which can becarried per OFDM symbol is less than half of the maximum number whichcan be carried by an OFDM symbol in an operating mode with the largestnumber of sub-carriers per OFDM symbol. As such, the symbol interleaver33.1 has been arranged to partition the interleaver memory 100. For thepresent illustration shown in FIG. 10 the interleaver memory then 100 isdivided into two parts 401, 402. As an illustration of the symbolinterleaver 33.1 operating in a mode in which data symbols are mappedonto the OFDM symbols using the odd interleaving process, FIG. 10provides an expanded view of each half of the interleaver memory 401,402. The expanded provides an illustration of the odd interleaving modeas represented for the transmitter side for four symbols A, B, C, Dreproduced from FIG. 4. Thus as shown in FIG. 10, for successive sets offirst and second data symbols, the data symbols are written into theinterleaver memory 401, 402 in a sequential order and read out inaccordance with addresses generated by the address generator 102 in apermuted order in accordance with the addresses generated by the addressgenerator as previously explained. Thus as illustrated in FIG. 10, sincean odd interleaving process is being performed for successive sets offirst and second sets of data symbols, the interleaver memory must bepartitioned into two parts. Symbols from a first set of data symbols arewritten into a first half of the interleaver memory 401, and symbolsfrom a second set of data symbols are written into a second part of theinterleaver memory 402, because the symbol interleaver is no longer ableto reuse the same parts of the symbol interleaver memory as can beaccommodated when operating in an odd and even mode of interleaving.

A corresponding example of the interleaver in the receiver, whichappears in FIG. 8 but adapted to operate with an odd interleavingprocess only is shown in FIG. 11. As shown in FIG. 11 the interleavermemory 540 is divided into two halves 410, 412 and the address generator542 is adapted to write data symbols into the interleaver memory andread data symbols from the interleaver memory into respective parts ofthe memory 410, 412 for successive sets of data symbols to implement anodd interleaving process only. Therefore, in correspondence withrepresentation shown in FIG. 10, FIG. 11 shows the mapping of theinterleaving process which is performed at the receiver and illustratedin FIG. 4 as an expanded view operating for both the first and secondhalves of the interleaving memory 410, 412. Thus a first set of datasymbols are written into a first part of the interleaver memory 410 in apermuted order defined in accordance with the addresses generated by theaddress generator 542 as illustrated by the order of writing in the datasymbols which provides a write sequence of 1, 3, 0, 2. As illustratedthe data symbols are then read out of the first part of the interleavermemory 410 in a sequential order thus recovering the original sequenceA, B, C, D.

Correspondingly, a second subsequent set of data symbols which arerecovered from a successive OFDM symbol are written into the second halfof the interleaver memory 412 in accordance with the addresses generatedby the address generator 542 in a permuted order and read out into theoutput data stream in a sequential order.

In one example the addresses generated for a first set of data symbolsto write into the first half of the interleaver memory 410 can be reusedto write a second subsequent set of data symbols into the interleavermemory 412. Correspondingly, the transmitter may also reuse addressesgenerated for one half of the interleaver for a first set of datasymbols for reading out a second set of data symbols which have beenwritten into the second half of the memory in sequential order.

Odd Interleaver With Offset

The performance of an interleaver, which uses two odd interleavers couldbe further improved by using a sequence of odd only interleavers ratherthan a single odd only interleaver, so that any bit of data input to theinterleave does not always modulate the same carrier in the OFDM symbol.

A sequence of odd only interleavers could be realised by either:

-   -   adding an offset to the interleaver address modulo the number of        data carriers, or    -   using a sequence of permutations in the interleaver        Adding an Offset

Adding an offset to the interleaver address modulo the number of datacarriers effectively shifts and wraps-round the OFDM symbol so that anybit of data input to the interleaver does not always modulate the samecarrier in the OFDM symbol. Thus the address generator, could optionallyinclude an offset generator, which generates an offset in an addressgenerated by the address generator on the output channel H(q).

The offset would change each symbol. For example, this offset couldprovide be a cyclic sequence. This cyclic sequence could be, forexample, of length 4 and could consist of, for example, prime numbers.For example, such a sequence could be:

0, 41, 97, 157

Furthermore, the offset may be a random sequence, which may be generatedby another address generator from a similar OFDM symbol interleaver ormay be generated by some other means.

Using a Sequence of Permutations

As shown in FIG. 5, a control line 111 extends from the control unit ofthe address generator to the permutation circuit. As mentioned above, inone example the address generator can apply a different permutation codefrom a set of permutation codes for successive OFDM symbols. Using asequence of permutations in the interleaver address generator reduces alikelihood that any bit of data input to the interleaver does not alwaysmodulate the same sub-carrier in the OFDM symbol.

For example, this could be a cyclic sequence, so that a differentpermutation code in a set of permutation codes in a sequence is used forsuccessive OFDM symbols and then repeated. This cyclic sequence couldbe, for example, of length two or four. For the example of the 16Ksymbol interleaver a sequence of two permutation codes which are cycledthrough per OFDM symbol could be for example:

8 4 3 2 0 11 1 5 12 10 6 7 9

7 9 5 3 11 1 4 0 2 12 10 8 6

whereas a sequence of four permutation codes could be:

8 4 3 2 0 11 1 5 12 10 6 7 9

7 9 5 3 11 1 4 0 2 12 10 8 6

6 11 7 5 2 3 0 1 10 8 12 9 4

5 12 9 0 3 10 2 4 6 7 8 11 1

The switching of one permutation code to another could be effected inresponse to a change in the Odd/Even signal indicated on the controlchannel 108. In response the control unit 224 changes the permutationcode in the permutation code circuit 210 via the control line 111.

For the example of a 1 k symbol interleaver, two permutation codes couldbe:

4 3 2 1 0 5 6 7 8

3 2 5 0 1 4 7 8 6

whereas four permutation codes could be:

4 3 2 1 0 5 6 7 8

3 2 5 0 1 4 7 8 6

7 5 3 8 2 6 1 4 0

1 6 8 2 5 3 4 0 7

Other combinations of sequences may be possible for 2 k, 4 k and 8 kcarrier modes or indeed 0.5 k carrier mode. For example, the followingpermutation codes for each of the 0.5 k, 2 k, 4 k and 8 k provide goodde-correlation of symbols and can be used cyclically to generate theoffset to the address generated by an address generator for each of therespective modes:

2 k Mode:

-   0 7 5 1 8 2 6 9 3 4 *-   4 8 3 2 9 0 1 5 6 7-   8 3 9 0 2 1 5 7 4 6-   7 0 4 8 3 6 9 1 5 2    4 k Mode:-   7 10 5 8 1 2 4 9 0 3 6 **-   6 2 7 10 8 0 3 4 1 9 5-   9 5 4 2 3 10 1 0 6 8 7-   1 4 10 3 9 7 2 6 5 0 8    8 k Mode:-   5 11 3 0 10 8 6 9 2 4 1 7 *-   10 8 5 4 2 9 1 0 6 7 3 11-   11 6 9 8 4 7 2 1 0 10 5 3-   8 3 11 7 9 1 5 6 4 0 2 10

For the permutation codes indicated above, the first two could be usedin a two sequence cycle, whereas all four could be used for a foursequence cycle. In addition, some further sequences of four permutationcodes, which are cycled through to provide the offset in an addressgenerator to produce a good de-correlation in the interleaved symbols(some are common to the above) are provided below:

0.5 k Mode:

-   3 7 4 6 1 2 0 5-   4 2 5 7 3 0 1 6-   5 3 6 0 4 1 2 7-   6 1 0 5 2 7 4 3    2 k Mode:-   0 7 5 1 8 2 6 9 3 4 *-   3 2 7 0 1 5 8 4 9 6-   4 8 3 2 9 0 1 5 6 7-   7 3 9 5 2 1 0 6 4 8    4 k Mode:-   7 10 5 8 1 2 4 9 0 3 6 **-   6 2 7 10 8 0 3 4 1 9 5-   10 3 4 1 2 7 0 6 8 5 9-   0 8 9 5 10 4 6 3 2 1 7    8 k Mode:-   5 11 3 0 10 8 6 9 2 4 1 7 *-   8 10 7 6 0 5 2 1 3 9 4 11-   11 3 6 9 2 7 4 10 5 1 0 8-   10 8 1 7 5 6 0 11 4 2 9 3-   *these are the permutations in the DVB-T standard-   **these are the permutations in the DVB-H standard

Examples of address generators, and corresponding interleavers, for the2k, 4k and 8k modes are disclosed in European patent application number04251667.4, which corresponds to U.S. Patent Application Publication No.2008/298487, the contents of which are incorporated herein by reference.An address generator for the 0.5k mode are disclosed in our co-pendingUK patent application number 0722553.5. Various modifications may bemade to the embodiments described above without departing from the scopeof the present invention. In particular, the example representation ofthe generator polynomial and the permutation order which have been usedto represent aspects of the invention are not intended to be limitingand extend to equivalent forms of the generator polynomial and thepermutation order.

As will be appreciated the transmitter and receiver shown in FIGS. 1 and7 respectively are provided as illustrations only and are not intendedto be limiting. For example, it will be appreciated that the position ofthe symbol interleaver and the de-interleaver with respect, for exampleto the bit interleaver and the mapper can be changed. As will beappreciated the effect of the interleaver and de-interleaver isun-changed by its relative position, although the interleaver may beinterleaving I/Q symbols instead of v-bit vectors. A correspondingchange may be made in the receiver. Accordingly the interleaver andde-interleaver may be operating on different data types, and may bepositioned differently to the position described in the exampleembodiments.

As explained above the permutation codes and generator polynomial of theinterleaver, which has been described with reference to animplementation of a particular mode, can equally be applied to othermodes, by changing the predetermined maximum allowed address inaccordance with the number of sub-carriers for that mode.

As mentioned above, embodiments of the present invention findapplication with DVB standards such as DVB-T, DVB-T2 (published as EN302 755) and DVB-H (published as ETSI EN 302 304 V 1.1.1 (2004-11)),which are incorporated herein by reference. For example embodiments ofthe present invention may be used in a transmitter or receiver operatingin accordance with the DVB-H standard, in hand-held mobile terminals.The mobile terminals may be integrated with mobile telephones (whethersecond, third or higher generation) or Personal Digital Assistants orTablet PCs for example. Such mobile terminals may be capable ofreceiving DVB-H or DVB-T compatible signals inside buildings or on themove in for example cars or trains, even at high speeds. The mobileterminals may be, for example, powered by batteries, mains electricityor low voltage DC supply or powered from a car battery. Services thatmay be provided by DVB-H may include voice, messaging, internetbrowsing, radio, still and/or moving video images, television services,interactive services, video or near-video on demand and option. Theservices might operate in combination with one another. In otherexamples embodiments of the present invention finds application with theDVB-T2 standard as specified in accordance with ETSI standard EN 302755. In other examples embodiments of the present invention findapplication with the cable transmission standard known as DVB-C2.However, it will be appreciated that the present invention is notlimited to application with DVB and may be extended to other standardsfor transmission or reception, both fixed and mobile.

The invention claimed is:
 1. A data processing apparatus configured tomap symbols received from a predetermined number of sub-carrier signalsof an Orthogonal Frequency Division Multiplexed (OFDM) symbol into anoutput symbol stream, the data processing apparatus, comprising: ade-interleaver configured to read-into a memory the predetermined numberof data symbols from the OFDM sub-carrier signals, and to read-out ofthe memory the data symbols into the output symbol stream to effect themapping, the read-out being in a different order than the read-in, theorder being determined from a set of addresses, with the effect that thedata symbols are de-interleaved from the OFDM sub-carrier signals, andan address generator configured to generate the set of addresses, anaddress being generated for each of the received data symbols toindicate the OFDM sub-carrier signal from which the received data symbolis to be mapped into the output symbol stream, the address generatorcomprising: a linear feedback shift register including a predeterminednumber of register stages and being configured to generate apseudo-random bit sequence in accordance with a generator polynomial, apermutation circuit configured to receive the content of the shiftregister stages and to permute the bits present in the register stagesin accordance with a permutation order to form an address of one of theOFDM sub-carriers, and a control unit configured in combination with anaddress check circuit to re-generate an address when a generated addressexceeds a predetermined maximum valid address, wherein the predeterminedmaximum valid address is approximately sixteen thousand, the linearfeedback shift register has thirteen register stages with a generatorpolynomial for the linear feedback shift register ofR′_(i)[12]=R′_(i−1)[0]⊕R′_(i−1)[1]⊕R′_(i−1)[4]⊕R′_(i−1)[5]⊕R′_(i−1)[9]⊕R′_(i−1)[11],and the permutation order forms, with an additional bit, a fourteen bitaddress R_(i)[n] for the i-th data symbol from the bit present in then-th register stage R′_(i)[n] in accordance with a code defined by thetable: R′_(i) bit positions 12 11 10 9  8 7 6  5 4 3 2  1  0 R_(i) bitpositions 8 4 3 2  0 11 1  5 12 10  6  7 
 9.


2. The data processing apparatus as claimed in claim 1, wherein thepredetermined maximum valid address is a value substantially betweentwelve thousand and sixteen thousand three hundred and eighty four. 3.The data processing apparatus as claimed in claim 1, wherein the OFDMsymbol includes pilot sub-carriers, which are arranged to carry knownsymbols, and the predetermined maximum valid address depends on a numberof the pilot sub-carrier symbols present in the OFDM symbol.
 4. The dataprocessing apparatus as claimed in claim 1, wherein the de-interleavermemory is arranged to effect the mapping of the received data symbolsfrom the sub-carrier signals onto the output data stream for even OFDMsymbols by reading in the data symbols according to a sequential orderand reading out the data symbols from memory according to the set ofaddresses generated by the address generator, and for odd OFDM symbolsby reading in the symbols into the memory in accordance with the set ofaddresses generated by the address generator and reading out the datasymbols from the memory in accordance with a sequential order.
 5. Thedata processing apparatus as claimed in claim 1, wherein the permutationcircuit is configured to change the permutation code, which permutes theorder of the bits of the register stages to form the addresses from oneOFDM symbol to another.
 6. The data processing apparatus as claimed inclaim 5, wherein the permutation circuit is configured to cycle througha sequence of different permutation codes for successive OFDM symbols.7. The data processing apparatus as claimed in claim 6, wherein thesequence of permutation codes comprises two permutation codes, which areR′_(i) bit positions 12 11 10 9 8 7 6  5 4 3 2 1  0 R_(i) bit positions8 4 3 2 0 11 1  5 12 10 6 7  9 and R′_(i) bit positions 12 11 10 9 8 76  5 4 3 2 1  0 R_(i) bit positions 7 9 5 3 11 1 4  0 2 12 10  8 
 6.


8. The data processing apparatus as claimed in claim 5, wherein for bothodd OFDM symbols and even OFDM symbols the interleaver is configured toread-into the memory the predetermined number of data symbols from theOFDM sub-carrier signals in accordance with the addresses generated bythe address generator, and to read-out from the memory the data symbolsinto the output symbol stream to effect the mapping in a sequentialorder.
 9. A receiver for receiving data from Orthogonal FrequencyDivision Multiplexing (OFDM) modulated signal, the receiver includingthe data processing apparatus according to claim
 1. 10. The receiver asclaimed in claim 9, wherein the receiver is configured to receive datawhich has been modulated in accordance with a Digital Video Broadcastingstandard including the Digital Video Broadcasting-Terrestrial, DigitalVideo Broadcasting-Handheld or the Digital VideoBroadcasting-Terrestrial2 standard.
 11. A method of mapping symbolsreceived from a predetermined number of sub-carrier signals of anOrthogonal Frequency Division Multiplexed (OFDM) symbol into an outputsymbol stream, the method, comprising: reading-into a memory thepredetermined number of data symbols from the OFDM sub-carrier signals,reading-out of the memory the data symbols into the output symbol streamto effect the mapping, the read-out being in a different order than theread-in, the order being determined from a set of addresses, with theeffect that the data symbols are de-interleaved from the OFDMsub-carrier signals, and generating the set of addresses, an addressbeing generated for each of the received symbols to indicate the OFDMsub-carrier signal from which the received data symbol is to be mappedinto the output symbol stream, the generating the set of addressescomprising: using a linear feedback shift register including apredetermined number of register stages to generate a pseudo-random bitsequence in accordance with a generator polynomial, using a permutationcircuit to receive the content of the shift register stages and topermute the bits present in the register stages in accordance with apermutation order to form an address, and re-generating an address whena generated address exceeds a predetermined maximum valid address,wherein the predetermined maximum valid address is approximately sixteenthousand, the linear feedback shift register has thirteen registerstages with a generator polynomial for the linear feedback shiftregister ofR′_(i)[12]=R′_(i−1)[0]⊕R′_(i−1)[1]⊕R′_(i−1)[4]⊕R′_(i−1)[5]⊕R′_(i−1)[9]⊕R′_(i−1)[11],and the permutation order forms, with an additional bit, a fourteen bitaddress R_(i)[n] for the i-th data symbol from the bit present in then-th register stage R′_(i)[n] in accordance with a code defined by thetable: R′_(i) bit positions 12 11 10 9  8 7 6  5 4 3 2  1  0 R_(i) bitpositions 8 4 3 2  0 11 1  5 12 10  6  7 
 9.


12. The method as claimed in claim 11, wherein the predetermined maximumvalid address is a value substantially between twelve thousand andsixteen thousand three hundred and eighty four.
 13. The method asclaimed in claim 11, wherein the OFDM symbol includes pilotsub-carriers, which are arranged to carry known symbols, and thepredetermined maximum valid address depends on a number of the pilotsub-carrier symbols present in the OFDM symbol.
 14. The method asclaimed in claim 11, wherein the using a permutation circuit to receivethe content of the shift register stages and permuting the bits presentin the register stages in accordance with the permutation code to forman address, includes changing the permutation code, which permutes theorder of the bits of the register stages to form the addresses, from oneOFDM symbol to another.
 15. The method as claimed in claim 14, whereinthe changing the permutation code, which permutes the order of the bitsof the register stages to form the addresses, from one OFDM symbol toanother includes cycling through a sequence of different permutationcodes for successive OFDM symbols.
 16. The method as claimed in claim15, wherein the sequence of permutation codes comprises two permutationcodes, which are R′_(i) bit positions 12 11 10 9 8 7 6  5 4 3 2 1  0R_(i) bit positions 8 4 3 2 0 11 1  5 12 10 6 7  9 and R′_(i) bitpositions 12 11 10 9 8 7 6  5 4 3 2 1  0 R_(i) bit positions 7 9 5 3 111 4  0 2 12 10  8 
 6.


17. The method as claimed in claim 14, wherein the reading-into thememory the predetermined number of data symbols from the OFDMsub-carrier signals, comprises for both odd OFDM symbols and even OFDMsymbols reading in the data symbols into memory in accordance with theaddresses generated by the address generator, and the reading-out of thememory the data symbols into the output symbol stream to effect themapping, comprises for both odd OFDM symbols and even OFDM symbolsreading-out the data symbols from the memory in a sequential order. 18.The method of receiving data from Orthogonal Frequency DivisionMultiplexing OFDM modulated symbols, the method comprising: receiving apredetermined number of data symbols from a predetermined number ofsub-carrier signals from the OFDM symbols for forming an output datastream, reading-into a memory the predetermined number of data symbolsfrom the OFDM sub-carrier signals, reading-out of the memory the datasymbols into the output symbol stream to effect the mapping, theread-out being in a different order than the read-in, the order beingdetermined from a set of addresses, with the effect that the datasymbols are de-interleaved from the OFDM sub-carrier signals, andgenerating the set of addresses, an address being generated for each ofthe received symbols to indicate the OFDM sub-carrier signal from whichthe received data symbol is to be mapped into the output symbol stream,the generating the set of addresses comprising: using a linear feedbackshift register including a predetermined number of register stages togenerate a pseudo-random bit sequence in accordance with a generatorpolynomial, using a permutation circuit to receive the content of theshift register stages and to permute the bits present in the registerstages in accordance with a permutation order to form an address, andre-generating an address when a generated address exceeds apredetermined maximum valid address, wherein the predetermined maximumvalid address is approximately sixteen thousand, the linear feedbackshift register has thirteen register stages with a generator polynomialfor the linear feedback shift register ofR′_(i)[12]=R′_(i−1)[0]⊕R′_(i−1)[1]⊕R′_(i−1)[4]⊕R′_(i−1)[5]⊕R′_(i−1)[9]⊕R′_(i−1)[11],and the permutation order forms, with an additional bit, a fourteen bitaddress R_(i)[n] for the i-th data symbol from the bit present in then-th register stage R′_(i)[n] in accordance with a code defined by thetable: R′_(i) bit positions 12 11 10 9  8 7 6  5 4 3 2  1  0 R_(i) bitpositions 8 4 3 2  0 11 1  5 12 10  6  7 
 9.


19. An address generator for use with reception of data symbolsinterleaved onto sub-carriers of an Orthogonal Frequency DivisionMultiplexed symbol, the address generator being configured to generate aset of addresses, each address being generated for each of the datasymbols to indicate one of the sub-carrier signals onto which the datasymbol is to be mapped, the address generator comprising: a linearfeedback shift register including a predetermined number of registerstages and being configured to generate a pseudo-random bit sequence inaccordance with a generator polynomial, a permutation circuit configuredto receive the content of the shift register stages and to permute thebits present in the register stages in accordance with a permutationorder to form an address, and a control unit configured in combinationwith an address check circuit to re-generate an address when a generatedaddress exceeds a predetermined maximum valid address, wherein thepredetermined maximum valid address is approximately sixteen thousand,the linear feedback shift register has thirteen register stages with agenerator polynomial for the linear feedback shift register ofR′_(i)[12]=R′_(i−1)[0]⊕R′_(i−1)[1]⊕R′_(i−1)[4]⊕R′_(i−1)[5]⊕R′_(i−1)[9]⊕R′_(i−1)[11],and the permutation order forms, with an additional bit, a fourteen bitaddress R_(i)[n] for the i-th data symbol from the bit present in then-th register stage R′_(i)[n] in accordance with the table: R′_(i) bitpositions 12 11 10 9  8 7 6  5 4 3 2  1  0 R_(i) bit positions 8 4 3 2 0 11 1  5 12 10 6  7  9.